Some rolling bearings for supporting e.g. rotary shafts of various rotary devices carry a rotational speed sensor for detecting the rotational speed (number of revolutions per unit time) of the rotary shaft. Such a rolling bearing with a rotational speed sensor includes a magnetic encoder comprising a metal core mounted to one of inner and outer races that rotates, and an annular magnetic member magnetized such that different magnetic poles are alternately arranged in the circumferential direction and fixed to the metal core, and a sensor element fixed to the stationary race for detecting changes in magnetic poles when the magnetic encoder rotates, thereby detecting the rotation of the rotating race.
The magnetic member of the magnetic encoder ordinarily comprises a magnetic material such as a ferrite and a binder such as a thermoplastic resin or rubber. The mixture of the magnetic material and the binder is formed to predetermined dimensions in a mold for e.g. injection molding. The thus formed magnetic member is fixed to the metal core by means of an adhesive or by pressing if the binder is a thermoplastic resin, or fixed to the metal core by vulcanization bonding if the binder is rubber (see JP patent publication 2002-349556A). The thus formed magnetic member is magnetized with a magnetizing device to form a desired number of magnetic poles.
Ordinarily, the sensor element is received in a sensor case mounted to an annular metal core fitted in or on the radially inner or outer surface of the stationary race at its end, and thus is mounted to the stationary race (as disclosed in JP patent publication 2002-295465A). In JP patent publication 2002-295465A, the sensor case is fixed to the metal core by adhesively bonding the former to the latter and engaging a protrusion formed on the metal core in a recess formed in the sensor case. The metal core is pressed into the radially inner surface of the outer race as the stationary race at its end. A substrate for the sensor element is also mounted in the sensor case. On the substrate, electric circuits for processing output signals of the sensor element are often mounted.
In some double row rolling bearings for supporting rotating members of medical devices and robots, in order to prevent vibrations of the rotating member, the inner race comprises two separate portions each having a raceway opposed to one of the two raceways of the outer race, with an axial gap defined therebetween, and an annular presser member is provided on one side of one of the two separate portions of the inner race to reduce the axial gap, thereby applying a preload to the bearing (as disclosed in JP patent publication 2000-329143A). It is preferable to provide such a double row bearing for supporting a rotating member of a medical device or a robot with a rotational speed sensor in order to detect the rotation of the rotating member.
In the arrangement in which the binder in the magnetic member is a thermoplastic resin, in order to manufacture a large-sized bearing having a magnetic encoder having a correspondingly large outer diameter, it is necessary to use a large-sized mold and a large-sized facility for injection molding, which increases the manufacturing cost. In the arrangement in which rubber is used as the binder and the magnetic member is fixed to the metal core by vulcanization bonding, a mold for keeping the magnetic member in contact with the metal core is necessary, and large-sized mold and facility for vulcanization bonding are necessary.
As disclosed in JP patent publication 2002-295465A, because in the sensor case in which the sensor element is mounted, a substrate on which electric circuits for processing output signals of the sensor element is also mounted in many cases, these elements are more likely to fail than the magnetic encoder due e.g. to vibrations. Because in such a rolling bearing with a rotational speed sensor in which the annular metal core to which the sensor case is mounted is pressed into or onto the radially inner or outer surfaces of the stationary race at its end, if the above-described failure occurs, the sensor case cannot be easily removed for repair or exchange. The annular metal core takes up extra space, so that it is difficult to provide a compact bearing.
If the rotational speed sensor disclosed in JP patent publication 2002-349556A is mounted on the double row rolling bearing with a preload application means as disclosed in JP patent publication 2000-329143A, because the double row rolling bearing itself comprises a large number of parts, the addition of the magnetic encoder and the sensor element of the rotational speed sensor excessively increases the number of parts of such a bearing. It is troublesome to assemble such a bearing.